transparent overlay over image
As requested the Blend file I used some time back has been placed here.



It's nothing more than a particle emitter with very few particles over a very long time - typically 50 particles over 1000 frames, their lifetime also 1000.

Physics settings were typically with a Size of 0.05, Brownian of 12.68, Damping of 0.224 .  

The resultant alpha layered output (single frame .png files) can be an overlay for a video or image.  They can also be adjusted for further visual effects in the compositor or VSE as one would any image or material.

Don't forget to nominate your own folder for the video output -
there'll be 1000 files going somewhere if you don't !

Glad you've got your project going as things are...